The Celtics are close enough to the luxury tax line to make a move or two and get below the line. In the past, the Celtics ownership group has made it clear that they would pay the tax for a contending team.
Here’s a blurb from an article by Keith Smith in 2018:
Celtics majority owner Wyc Grousbeck recently said “We’re hoping to pay for performance. We’re prepared to do whatever is takes to win again”. Celtics fans have no reason to doubt Grousbeck on the willingness to spend. Ownership paid the tax for five straight seasons from 2008 through 2013 to keep the Pierce/Garnett window open for as long as they could; they paid over $47 million in tax alone, on top of regular salary, to do so.